The present invention is directed to the detection of dry/wet states of a thermistor bead, in general, and more particularly, to a method and apparatus for detecting a dry/wet state of a thermistor bead based on a difference in voltage across the thermistor bead in response to a difference in current conducted therethrough and on thermistor bead state transition.
Sensing liquid level in a container, like fuel in an aircraft fuel tank, for example, has been performed using resistive bead type thermistors which have an inverse temperature coefficient. When current is conducted through a thermistor bead in air, i.e. a dry state, the bead increases in temperature and exhibits a low resistance to the current. In contrast, when the thermistor bead is submersed in a liquid, like jet fuel, for example, the bead is cooled and its resistance to current conducted therethrough is increased. Current vs. Voltage (I/V) characteristics of a typical thermistor bead at various temperatures is shown in the graph of FIG. 1. It is readily observable from the graph of FIG. 1 that the voltage across the thermistor becomes a viable measurement for detecting a wet vs. dry (wet/dry) state of the thermistor bead as the current conducted therethrough becomes greater than 45 milliamps. For example, by passing a constant current through a thermistor bead and lowering the bead from the top of the container, it may be determined at what level in the container the thermistor bead becomes submersed into the liquid by detecting the wet/dry state thereof based on the voltage across the thermistor bead.
Known interface circuitry which uses the above described method of detecting the dry/wet state of a thermistor bead is shown in the block diagram schematic of FIG. 2. Referring to FIG. 2, a thermistor bead 10 is coupled between a constant current source 12 and a common or ground return. The constant current source 12 is powered by a power supply and is operative to conduct current through the thermistor bead 10. The voltage across the thermistor bead 10 is sensed by one input of a comparator circuit 16 which is also powered by the supply 14 and common return. A reference voltage is generated by a circuit 18 which is powered by the supply 14. The comparator circuit 16 compares the reference voltage which is coupled to another input thereof with the thermistor bead voltage. When the thermistor bead voltage exceeds the reference voltage, a wet bead state is effected at the output of the comparator 16 and when the thermistor bead voltage is less than the reference voltage, a dry bead state is effected at the output of the comparator 16.
As noted above, to insure proper performance of the thermistor bead and detection circuitry using the above described method, the bead 10 should be biased with a constant current equal to or greater than 45 milliamps where voltage levels across the bead are dry/wet distinct for all practical temperature environments (see the graphs of FIG. 1, for example). For level sensing of combustible liquids in a container with a thermistor bead, the bias current level of 45 milliamps may not be considered safe, and thus unacceptable. For example, the FAA has deemed this bias current level unacceptable in terms of the maximum allowable current that may enter an aircraft fuel tank. Only currents less than 30 milliamps with justification have been deemed acceptable for aircraft fuel tanks. Accordingly, a thermistor bead may not be an acceptable level measurement sensor for combustible liquids in all cases using interface circuitry implementing the above described traditional method.
In accordance with one aspect of the present invention, a method of detecting a dry/wet state of a thermistor bead comprises the steps of: generating a current waveform that transitions between first and second current levels; conducting the current waveform through the thermistor bead; measuring a difference in voltage across the thermistor bead in response to the first and second current levels; and detecting the dry/wet state of the thermistor bead based on the measured difference in voltage.
In accordance with another aspect of the present invention, apparatus for detecting a dry/wet state of a thermistor bead comprises: a first circuit coupled to the thermistor bead for generating a current waveform that transitions between first and second current levels and for conducting the current waveform through the thermistor bead; a second circuit coupled to the thermistor bead for measuring a difference in voltage across the thermistor bead in response to the first and second current levels; and a third circuit coupled to the second circuit for detecting the dry/wet state of the thermistor bead based on the measured difference in voltage.
In accordance with a further aspect of the present invention, a method of detecting a transient dry/wet state of a thermistor bead comprises the steps of: conducting a current waveform through the thermistor bead; measuring a voltage waveform across the thermistor bead in response to the current waveform; differentiating the voltage waveform to produce a voltage pulse representative of a transition of the voltage waveform from one voltage potential to another; and detecting the transient dry/wet state of the thermistor bead based on the voltage pulse.
In accordance with yet another aspect of the present invention, apparatus for detecting a transient dry/wet state of a thermistor bead comprises: a first circuit coupled to the thermistor bead for conducting a current waveform therethrough; a second circuit coupled to the thermistor bead for measuring a voltage waveform across the thermistor bead in response to the current waveform and differentiating the voltage waveform to produce a voltage pulse representative of a transition of the voltage waveform from one voltage potential to another; and a third circuit coupled to the second circuit for detecting the transient dry/wet state of the thermistor bead based on the voltage pulse.
In accordance with yet another aspect of the present invention, a method of detecting a dry/wet state of a thermistor bead comprises the steps of: generating a current waveform that transitions between first and second current levels; conducting the current waveform through the thermistor bead; measuring a voltage waveform across the thermistor bead in response to the current waveform; measuring a slope of the voltage waveform in response to the first and second current levels and generating a first signal indicative of the measured slope; differentiating the voltage waveform to produce a voltage pulse representative of a transition of the voltage waveform from one voltage potential to another and generating a second signal indicative of the polarity of the voltage pulse; and detecting the dry/wet state of the thermistor bead based on the first and second signals.
In accordance with yet another aspect of the present invention, apparatus for detecting a dry/wet state of a thermistor bead comprises: a circuit coupled to the thermistor bead for generating a current waveform that transitions between first and second current levels and conducting the current waveform through the thermistor bead; a circuit coupled to the thermistor bead for measuring a voltage across the thermistor bead in response to the current waveform and generating a voltage waveform signal representative thereof; a first detector circuit, coupled to the voltage measuring circuit, for determining a slope of the voltage waveform signal in response to the first and second current levels and generating a first signal indicative of the determined slope; a second detector circuit, coupled to the voltage measuring circuit, for differentiating the voltage waveform signal to produce a voltage pulse representative of a transition of the voltage waveform signal from one voltage potential to another and generating a second signal indicative of the polarity of the voltage pulse; and
a logic circuit coupled to the first and second detector circuits for detecting the dry/wet state of the thermistor bead based on the first and second signals generated thereby.